1. Field of the Invention
The present invention relates to a transmitting device capable of transmitting packets of, for example, video content data via a network, a receiving device capable of receiving the packets, a method of transmitting packets with the transmitting device, a method of receiving packets with the receiving device, and programs for the same.
2. Description of the Related Art
A transmission mode for an Internet Protocol (IP) network according to the related art includes unicast transmission in which a device on the transmission side transmits packets to a device on the reception side by assigning a single destination address and multicast transmission in which packets are transmitted to a plurality of devices on the reception side on the basis of a plurality of grouped destination addresses. Unicast transmission is generally suitable for reliably transmitting relatively small amounts of data, such as files, whereas multicast transmission is suitable for a streaming method in which large amount of data, such as audio and video (AV) contents using, for example, the Internet or a Local Area Network (LAN), are delivered to a plurality of devices on the reception side.
In unicast transmission, transmission control protocol (TCP), which is a higher-level layer IP, is also used to secure reliability, and packets are transmitted after the device on the transmission side adds error detection codes to the packets. When errors occur in the transmitted packets, the errors are detected by the device on the reception side by the error detection codes, the packets are discarded, a process for requesting the device on the transmission side to resend the packets (automatic repeat request (ARQ)) is carried out, and the errors are corrected by the device on the transmission side resending the packets on the basis of this request.
In multicast transmission, a user datagram protocol (UDP), which is a higher-level layer IP, is also used to improve the transmission efficiency. However, since TCP cannot be used, retransmission control by ARQ cannot be carried out. Therefore, in multicast transmission, the device on the transmission side adds error code signals, and when errors occur, the error correction codes are used by the reception side device to detect and correct the errors without resending the packets (this is known as forward error correction (FEC)).
In this way, by using ARQ in unicast transmission, even when errors are included in all sets of data, the errors in the sets of data can be corrected. However, in multicast transmission, if multiple errors occur, the errors cannot be completely corrected even by using an FEC according to the related art.
The error-generation characteristic changes depending on the characteristic of the transmission path. For example, in general, the probability of errors occurring in a line transmission path is extremely low, and the errors occur randomly. In contrast, the probability of errors occurring in a wireless transmission path varies, and there is a high probability of the errors occurring in bursts. However, on the Internet, the path of data transmission cannot be assigned, and thus, errors cannot be predicted. Therefore, in many cases employing multicast transmission, errors cannot be completely corrected using error correction signals added to the packets at the device on the transmission side by the FEC according to the related art.
Consequently, it is desirable to carry out retransmission in multicast transmission so as to reliably correct errors. However, as described above, retransmission cannot be carried out in multicast transmission according to the related art. To carry out retransmission in unicast transmission by employing ARQ, an entire packet is discarded for, for example, an error of merely one bit, causing a significant reduction in the transmission efficiency.
Patent Document 1, mentioned below, describes a communication method as a technology that enables retransmission during multicast transmission. According to this communication method, when multicast transmission that does not include retransmission is being carried out between a wireless station and a wireless terminal, the quality of the wireless transmission is determined at the wireless station or the wireless terminal; when the quality of the wireless transmission becomes low, the wireless terminal is separated from the multicast group and communication between the wireless station and the wireless terminal is switched to unicast transmission including retransmission; when carrying out unicast transmission, the quality of the wireless transmission is determined at the wireless station or the wireless terminal on the basis of the data received by the wireless terminal; when it is determined that the quality of the wireless transmission is good, the wireless terminal is returned to the multicast group; and the communication unit is switched to multicast transmission that does not include retransmission.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2001-308784 (paragraphs [0010] and [0012], FIG. 7)